1. Field of the Invention
The present invention relates to a display device. In particular, the invention relates to a display device with color reproducibility of high fidelity by reducing chromaticity difference in white color display caused by difference in optical property for each pixel.
2. Description of the Prior Art
In an active type flat panel type display device for performing on-off control for each pixel, a display panel is combined with the surrounding circuits and with the structural members as required. It is generally practiced that the display panel is provided by arranging a switching element represented by a thin-film transistor (TFT) and a multiple of pixels with pixel electrodes driven by TFT arranged in rows and columns in matrix form on an insulating substrate.
A plurality of gate lines to supply scan signals to select a multiple of TFTs arranged in matrix form for each row and a plurality of data lines to supply display data to TFT connected to the selected gate lines are arranged in crossover arrangement in matrix form to match the rows and columns. These lines are the so-called thin-film lines. Each pixel is arranged at an intersection of the thin-film lines (gate lines and data lines). In addition to the gate lines and data lines, some of the display devices have thin-film lines necessary to match the display method of the display device. The following description can also be applied to such type of thin-film lines.
A typical example of this type of display provided with pixel electrode for each pixel is a liquid crystal display device. In addition, an organic EL display device is also known. Description will be given below by taking an example on the liquid crystal display device. FIG. 17 is a schematical cross-sectional view of a panel to explain a conventional type liquid crystal display device. This can also be applied to the organic EL display device in a view-point of transparent electrodes being used.
The liquid crystal display device is provided by placing a liquid crystal LC between a first substrate SUB1 and a second substrate SUB2, each made of an insulating plate, for which glass is used as a suitable material. On inner surface of the first substrate SUB1, data signal line, scan signal line, thin-film transistor TFT, etc. are formed (not shown). The pixel electrodes PXR, PXG, and PXB of three colors (red, green, and blue) under on-off control by thin-film transistor are provided, and on an upper layer of these, a first orientation film ORI1 is formed. The first substrate SUB1 is also called a thin-film transistor substrate (TFT substrate).
On the other hand, on inner surface of the second substrate SUB2, color filters RF, GF, and BF of three colors (red, green, and blue) are formed to match the pixel electrodes PXR, PXG, and PXB of the first substrate SUB1 respectively. A counter electrode (common electrode) AT is formed by all over deposition to cover the color filters RF, GF, and BF, and a second orientation film ORI2 is formed on it. This second substrate SUB2 is also called a color filter substrate (CF substrate).
The pixel electrodes PXR, PXG, and PXB and the counter electrode (common electrode) AT are made of transparent conductive films, for which ITO is used as a suitable material. Refractive index and film thickness of each of the pixel electrodes PXR, PXG, and PXB and the counter electrode AT are the same regardless of the pixel. For instance, in case of ITO, when refractive index is 2.0 and film thickness is 130 nm, transmissivity is 94.98%. To standard white color on CIE 1931 xy chromaticity coordinates, chromaticity difference is 0.00441. FIG. 18 shows CIE 1931 xy chromaticity coordinates.
The chromaticity difference as described above is defined as “a distance on coordinates to standard color of CIE 1931 xy chromaticity coordinates”. Standard white color indicates chromaticity coordinates (xw, yw)=(0.333, 0.333) obtained from spectrum, which has 100% transmissivity in all-wavelength range. Chromaticity difference of a certain point (xy) on FIG. 18 is expressed by the distance ΔL from the coordinates (xw, yw). That is, ΔL={(x−xw)2+(y−yw)2}1/2.
As a general theory on chromaticity difference, the following facts are known. Specifically, when chromaticity difference is 0.004 or more, color difference is recognizable. However, the recognition of the color difference is dull in the direction of blue (B) (color difference is difficult to recognize even when chromaticity difference exceeds 0.004), and recognition is sharp in the direction of yellow (Y) (color difference is easily recognized even when chromaticity difference is less than 0.004).
The Patent Document 1 discloses a liquid crystal display device, in which poor brightness uniformity in display caused by the difference of interference spectrum at peak wavelength of the light source is suppressed by controlling refractive index and film thickness of transparent electrode, which constitutes the pixels.
[Patent Document 1] JP-A-4-166915